Multi-State work: The European Court of Justice strictly defines a substantial part of the activities
On 4 September 2025, the European Court of Justice, in the case of Hakamp (C-203/24), gave a strict interpretation of the concept of ‘substantial part’ under Article 13 of Regulation 883/2004 (‘pursuit of activities in two or more Member States’). The Court clarified that, when assessing whether a worker carries out a substantial part of their activities in the State of residence, the threshold of 25% of working time and/or remuneration in that State must be met. Other circumstances or criteria may not be taken into account in determining whether a substantial part of the activities is carried out in the State of residence.
In this case, a skipper worked in several Member States, of which approximately 22% took place in his State of residence, the Netherlands. Although he carried out less than 25% of his activities in the Netherlands, an A1 certificate was issued confirming the application of the Dutch social security system. The skipper lodged an objection against this decision with the Social Security Bank (SVB). The SVB rejected the objection, ruling that there was indeed a substantial part of the activities in the Netherlands, since the boat was registered there and both the owner and the operator were established in the Netherlands. Other appeal bodies confirmed this view: a worker who performs less than 25% of their activities in the State of residence may nonetheless be regarded as carrying out a substantial part of their activities there if sufficient additional indicators support such a conclusion.
Ultimately, the skipper brought the case before the Supreme Court of the Netherlands, which referred several preliminary questions to the Court of Justice. The central issue was whether, in assessing whether a person who performs salaried activities in two or more Member States carries out a substantial part of their activities in the Member State of residence, account may be taken of circumstances other than the 25% share of working time and/or remuneration. The Court of Justice replied that, in such an assessment, it must in principle be determined whether at least 25% of the employee’s working time and/or remuneration takes place or is received in the State of residence. Other circumstances or criteria may not be taken into account in this context. The Court thus confirmed the strict application of the 25% threshold in defining the criterion of a substantial part of the activities.
The Supreme Court of the Netherlands also asked which period should be taken into account when assessing whether a worker carries out a substantial part of their activities in the State of residence. The Court of Justice clarified that the relevant reference period is the expected situation of the worker over the following twelve calendar months. According to the Court, this period begins at the point at which the performance of activities in two or more Member States begins.
Key message
The Court emphasised that, in assessing the ‘substantial part’ of activities, the 25% threshold of working time and/or remuneration is decisive. Other factors may not be taken into account. Employers and employees should be aware that, in cases of simultaneous employment in two or more Member States, once the 25% threshold is reached in the State of residence, the social security legislation of that State applies, unless the Framework Agreement can be invoked (see more about this in our Newsflash).